1. Field of the Invention
The present invention relates to a method of synchronizing base stations of a terrestrial cellular communication system using a satellite component. The objective of synchronizing the base stations is to locate or to assist with the location of the terminals of said communication system. Synchronization may be exploited in the use of techniques for assisting satellite positioning or techniques that work by triangulation of measurements effected directly on the signals transmitted by the base stations.
2. Description of the Prior Art
In a satellite positioning system utilizing GNSS (Global Navigation Satellite System) type receivers such as a GPS or Galileo receiver, the data signals enabling the receiver to calculate its position come from different satellites (at least four to determine the four unknowns x, y, z and t). It is well known by the man skilled in the art that positioning by such a system presupposes two types of processing in the receiver.
The first consists in acquiring the signal coming from at least four satellites, the second consists in evaluating the distance separating the receiver from the satellites from which the signal has been received. The interface between a radio navigation satellite system (RNSS) and the user receiver relies on a radio signal itself relying on a spread spectrum technique well known to the man skilled in the art. Spread spectrum techniques, in their most routine form such as the C/A code GPS, F/Nav Galileo, rely on the use of a periodic pseudo-random code. In the context of the GPS, that code has a period of 1 millisecond (ms). This code is added to a digital navigation message, that message including a certain number of items of information necessary for the calculation of the position of the receiver, typically:                a time reference, well known in GPS under the name TOW (Time of Week) corresponding to the time of sending of the message,        the position of the satellite at the time of the sending of the message, well known in GPS under the name ephemerides,        certain corrections to be made to the onboard clock of the satellite, well known in GPS under the name clock corrections, aiming to correct the time reference relative to the global clock of the system,        certain propagation correction parameters, such as parameters for correction of the propagation speed of electromagnetic waves in the layers of the atmosphere (in particular the ionosphere),        the approximate position of the other satellites of the constellation via data known as almanacs.        
The data bit rate is of course lower than the periodic spreading code bit rate. In the GPS SPS (GPS Satellite Positioning System) signal, the data bit rate typically rises to 50 bits per second, whereas that of the code is 1.023 million chips per second. A complete code being constituted of 1023 chips (i.e. 1 ms). All of the data added modulo 2 to the spreading code is transmitted on a carrier. In GPS, the carrier is typically at 1.57542 GHz.
The determination of the position of the receiver is represented in FIG. 1. The principle consists in a receiver [4] determining the distance separating it from at least three satellites of the constellation referenced by satellites [1], [2] and [3] (three satellites for location in two dimensions and four satellites for location in three dimensions). Once these distances [d1], [d2] and [d3] have been determined, the receiver can determine its position at the intersection of the spheres whose center is the position of each of the satellites themselves and whose radius is given by the distance [di]. The distance measurement is effected by measuring the time of arrival of a radio signal coming from the satellite. It follows that the essential information coming from the satellite via the navigation message that the receiver must process consists of the pair (sending TOW, position of the satellite at the time of sending). The satellite transmits in its navigation message its ephemerides (Keplerian parameters) enabling the receiver to calculate the position of the satellite in a frame of reference linked to the Earth. In the GPS case the ephemerides consist of 16 parameters.
M0Mean anomalyDnMean displacementEEccentricity(A)1/2Root of half major axisOMEGA 0Longitude of ascending nodeI0InclinationWArgument of perigeeOMEGA DOTTime derivative of right ascensionI DOTTime derivative of inclinationCucCosine amplitude of harmonic of latitudeargument correction termCusSine amplitude of harmonic of latitudeargument correction termCrcCosine amplitude of harmonic of orbitradius correction termCrsSine amplitude of harmonic of orbitradius correction termCicCosine amplitude of harmonic ofinclination angle correction termCisSine amplitude of harmonic ofinclination angle correction term
These parameters are repeated every 30 seconds in the navigation message.
The position of the satellite being obtained, it remains for the receiver to detect the time of sending of the message in order to deduce the propagation time of the wave and then the distance separating it from the satellite, and thus the radius of one of the three necessary spheres. As indicated hereinabove, the time also forms part of the content of the navigation message broadcast by the satellite. That time is repeated every 6 seconds. However, it is necessary to apply a satellite clock correction to the time read in the navigation message in order to transpose the transmitted time into a system reference common to all the satellites. This correction is transmitted every 30 seconds.
In conclusion, it is clearly apparent that a receiver can be in a position to determine its position only at the end of a minimum time of 30 seconds after having acquired the signal. The acquisition of the signal means the whole of the first operation to be effected by the receiver, which enables it to be synchronized in frequency and in time to the bit streams transmitted, an essential phase for the demodulation of the navigation message. For the receiver, acquisition consists in effecting a time-frequency search of the energy of the signal coming from the satellite. Locking onto the frequency of the signal from the satellite consists for the receiver in being tuned to the frequency at which the signal from the satellite is received. The receiver has three uncertainties leading it to effect this search:                Doppler effect linked to the mobility of the satellite,        Doppler effect linked to the mobility of the user,        uncertainty linked to the accuracy of the receiver clock.        
For the receiver, time locking consists in identifying a code transition in the received signal. The spreading code in the case of the GPS being periodic with a period of 1 ms, the time search is effected with a 1 ms horizon. Once the code transition has been identified, the man skilled in the art knows how to identify a bit transition and then the frame synchronization broadcast in the navigation message.
This time-frequency search is very costly in terms of receiver complexity and limits commensurately the performance of the receiver.
To summarize, the time taken by a receiver to provide a first position is constrained by a very costly first phase of seeking time-frequency synchronization and also by the reading of basic information in the navigation signal (greater than 30 s).
The method known to the man skilled in the art for alleviating this problem is known as Assisted GPS or Assisted GNSS. This method consists in coupling a cellular telecommunication system and a satellite navigation signal receiver. This method is described in FIG. 2. It assumes that the satellite navigation signal receiver is coupled to a cellular telecommunication receiver (terminal) [11]. A network equipment commonly called the assistance data server [8] listens continuously to the satellites of the satellite constellation via a radio signal [6a] and a control antenna referenced [7]. The information from the navigation message from each satellite is then stored by the server [8]. When the receiver [11] is searching for its position, it requests a certain number of items of assistance data by means of a call via a base station [10] of the cellular network [9] to the assistance data server. The assistance data is then returned by the server [8] to the receiver [11] via the base station [10]. This assistance data facilitates the processing of the signal [6b] received by the receiver [11] coming from the satellite [5] and confers on the receiver performance that is enhanced, inter alia, in terms of calculation time. In fact, the assistance data may be of the following type:                Content of the navigation message broadcast in the signals [6b] and [6a]. The content is returned at a bit rate much higher than the bit rate of the navigation message. The time taken to route the data essential to the determination of the position is therefore changed from 30 seconds to 1 to 2 seconds.        Pre-location of the receiver [11]. In fact, the receiver [11] being connected to the base station [10], the server [8] is in a position to know that the receiver is in the vicinity of the base station [10]. In a GSM type network, the dimension of the cells is typically less than 35 km.        A time reference. The server [8] receiving the data from the satellite [5] is in a position to know the satellite system time and therefore to broadcast it to the receiver [11]. Most cellular communication networks being asynchronous, the time reference transmitted can achieve an accuracy only of the order of 2 to 3 seconds.        Different types of corrections: propagation speed corrections, satellite onboard clock correction, local propagation correction, etc.        
The knowledge of a pre-location, of the ephemerides of the satellites and of an approximate time reference enables the receiver to calculate the Doppler effect of the satellites in view, greatly reducing the uncertainty in terms of frequency to be swept during the acquisition phase. Similarly, the ephemerides of the satellites being known via the call to the server [8], it becomes unnecessary for the receiver [11] to demodulate this data in the navigation message [6b], which eliminates the constraint of 30 seconds previously highlighted for calculating the position. It then suffices for the receiver to determine a time event in the signal [6b] from the satellite, in other words to find the spreading code transition and then the transmitted time, the TOW in the GPS signal which recurs every 6 seconds. There is therefore clearly a significant improvement in performance, as much with regard to the time necessary for the calculation of a location point as with regard to the sensitivity. Sensitivity means the minimum power of the signal received by the receiver enabling it to perform adequate processing.
The assistance example is provided in a GSM type cellular network, it goes without saying that it may be extended to other systems such as WIFI, WIMAX type systems.
All the above communication systems are asynchronous, that is to say the base stations or access point do not have the same time reference, although they have a very stable clock, namely 5.10−9 according to the GSM (Global System for Mobile communications) standard. As indicated hereinabove, it is very difficult for an equipment of the communication network to provide accurately timed assistance to a terminal comprising a GNSS receiver coupled to a telecommunication transceiver. By “accurate” is meant a time accuracy better than one millisecond, enabling the terminal to find more easily a code transition in the satellite signal without ambiguity.
Moreover, the asynchronism of the base stations prevents the use of triangulation techniques on the telecommunication signals, well known to the man skilled in the art under the name TOA (Time of Arrival) or EOTD (Enhanced Observed Time Difference). Those techniques are derived from the location method employed on GPS type satellite systems by substituting the base stations for the satellites. The major difference therefore lies precisely in the fact that the GPS satellites are synchronous whereas the terrestrial base stations are not, rendering unsuccessful and impossible the techniques employed without an indispensable modification of the cellular telecommunication network.
A solution consists in equipping each access point with a GNSS or GPS device capable of sending to the equipment of the network the time offset (time shift) between the base station and the satellite location system. This method amounts to equipping the telecommunication network with location measurement units (LMU). It is known that such a device, in addition to calculating its position, determines time information with an accuracy better than one nanosecond. This information from the device is then used to time stamp the start of the frames sent by the base station equipped with the reference device. In fact, the system of frames transmitted by a base station is very regular and, according to the GSM standard, conforms to the representation given in FIG. 3. The message in fact consists of an uninterrupted succession of hyperframes [12], themselves consisting of superframes [13], themselves consisting of frames [14] and then time slots [15]. This regular succession enables logical channels used by each of the terminals and the services of the cellular communication network to be defined. It follows equally that the frame structure, because the clocks of the base stations are extremely accurate and regular, supplies stable timing information usable from the moment that one of the frame change events is identified relative to a universal time reference such as that provided by a GNSS or GPS system. In the system described hereinabove based on the use of LMU, the GPS or GNSS device equipping the base station fulfills precisely this function of providing a universal time reference. The information being centralized by an equipment of the network, it may be used to provide a terminal of the communication network with extremely accurate time information, of the order of one nanosecond.
This solution is extremely costly because of the deployment cost and the maintenance cost of such a system.